Aircraft interaction with electric field of thundercloud and observations of hard radiation

The purpose of this thesis is to study the relationship between the different phases of lightning flashes and the production of high-energy emissions, as well as highenergy emissions observed from thunderclouds. The data consists of local electric field-, current- and energy measurements on board an aircraft. The lightning attachment points on the aircraft, were identified by the local electric field signatures of the positive and negative leader initiations. Showing a preference of attachment points based on aircraft model and/or flight altitude. Four distinct radiation producing events were identified by the electric fields and x-ray counts measured during the flights. Single photons in association with the stepping of negative leaders, with photon energies close to 100 keV. A correlation between the changes in local electric field, current and the energy of the observed photons were investigated, but were not found. Bursts of photons in association with recoils were observed. Recoils are microsecond fast changes in the local electric field, observed during the return strokes, dart leaders and recoil streamers of a lightning flash. The energies of the observed photon bursts are similar to that of the single photons observed during negative leader steps. Minute long gamma-ray glows from the data set has previously been reported. The findings were used as a basis to search for more gamma-ray glows in the dataset, and a total of 7 new low intensity gamma-ray glow candidates were found. A statistical analysis of the count rates were done for all observed gamma-ray glows to compare the events. Millisecond long x-ray count rate enhancements were observed on 14.01.2016. The lightning flashes were analysed by examining x-ray count rates and local electric fields. The energy spectrum of some of the observed flashes revealed an enhancement at 511 keV, which is the signature energy for positron annihilation. Two previously observed events were compared to two new findings, showing a very good correlation.Masteroppgave i fysikkMAMN-PHYSPHYS39

Aircraft and spacecraft observations of high-energy radiation associated with lightning leaders

Denne avhandlingen undersøker røntgen og gammastråling assosiert med lyn ledere, observert av instrumenter på fly og fra verdensrommet. Avhandlingen består av tre forskningsartikler, publisert i Journal of Geophysical Research: Atmospheres. I den første delen blir data av lyn initiert av fly, fra tre fly kampanjer brukt sammen med tre forskjellige modeller. Fra analyse av de initierte lynene blir begrensninger på såkalte recoil leaders bestemt. Ved å bruke data fra flere forskjellige instrumenter i verdensrommet, kombinert med radio atmospherics målt fra bakken, og optisk data fra Atmosphere-Space Interactions Monitor (ASIM) finner vi at Terrestrial Gamma-ray Flashes (TGFs) blir produsert i starten av lynet. En økning av radio atmospherics er også observert noen hundre millisekund etter TGFene. Høy energi og optisk data fra ASIM ble også brukt til å identifisere en gruppe med TGFer som var assosiert med optiske signaler. Fra en analyse av disse hendelsene bestemmes sekvensen av TGF og optisk lys. Det ble også funnet at TGFer med lengre varighet har lengre tid mellom starten av TGFene og det optiske lyset.This thesis investigates high-energy radiation associated with lightning leaders, observed by aircraft and space-borne instruments. The thesis consists of three papers published in the Journal of Geophysical Research: Atmospheres. In the first part, data of aircraft-triggered lightning from three flight campaigns were used together with three different models. From an analysis of the events, constraints to the properties of so-called recoil leaders were determined. Using data from several different space-borne instruments combined with ground-detected radio atmospherics and optical data from the Atmosphere-Space Interactions Monitor (ASIM) it is determined that Terrestrial Gamma-ray flashes (TGFs) are produced at the beginning of the lightning flash. An increase in detected radio atmospherics is also observed a few hundred milliseconds after the TGFs. High-energy and optical data from ASIM was used to identify a sample of TGFs found to be associated with observed optical pulses. From an analysis of these events, the sequence of TGF-optical light was determined. A tendency for long-duration TGFs to have longer delays between onset of TGF and optical light was also determined.Doktorgradsavhandlin

In‐Flight Observation of Positron Annihilation by ILDAS

We report a 511‐keV photon flux enhancement that was observed inside a thundercloud and is a result of positron annihilation. The observation was made with the In‐flight Lightning Damage Assessment System (ILDAS) on board of an A340 test aircraft. The aircraft was intentionally flying through a thunderstorm at 12‐km altitude over Northern Australia in January 2016. Two gamma ray detectors showed a significant count rate increase synchronously with fast electromagnetic field variations registered by an on‐board antenna. A sequence of 10 gamma ray enhancements was detected, each lasted for about 1 s. Their spectrum mainly consists of 511‐keV photons and their Compton component. The local electric activity during the emission was identified as a series of static discharges of the aircraft. A full‐scale Geant4 model of the aircraft was created to estimate the emission area. Monte Carlo simulation indicated that the positrons annihilated in direct vicinity or in the aircraft body.publishedVersio

The Temporal Relationship Between Terrestrial Gamma-Ray Flashes and Associated Optical Pulses From Lightning

We present 221 Terrestrial Gamma-ray Flashes (TGFs) and associated optical pulses observed by the Atmosphere-Space Interactions Monitor (ASIM) on board the International Space Station. The events were detected between the end of March 2019 and November 2020 and consist of X- and gamma-ray energy detections, as well as photometer data (180–230, 337, and 777 nm) and optical camera data (337 and 777 nm). Using the available ASIM data and applying a consistency check based on TGF characteristics and lightning detections from lightning radio atmospherics close in time, we determine the most likely position of the TGFs in relation to the photometer field of view (FoV), and the association to the observed optical pulses. Out of the 221 events we find 72 events where the TGF and optical data are determined to be associated and inside the photometer FoV. Using the measured TGF durations and the time between the onsets of the TGFs and optical pulses we find: (a) That the TGF onsets are always before or at the same time as the optical pulse onsets (taking into account cloud scattering). (b) A tendency for longer duration TGFs to have longer delays between onsets. (c) Two groups of events: (a) where there is a possible overlap between the TGFs and the optical emissions, as the TGFs last longer than the delay between onsets and (b) where the TGFs and optical emissions do not overlap, as there are long delays between the onsets, which cannot be explained by cloud scattering.publishedVersio

Constraints on Recoil Leader Properties Estimated from X-ray Emissions in Aircraft-Triggered Discharges

During Airbus aircraft campaigns flying into thunderstorms in 2014–2016, X-rays were observed during two stages of aircraft-triggered lightning: nanosecond pulses of X-rays associated with negative leader steps and bursts of X-rays during recoil events. This work will focus on the observations of X-ray bursts associated with recoils. Recoils are observed as microsecond-fast changes in the local electric field, associated with large currents passing through the aircraft, and are found to sometimes be associated with bursts of X-rays. From over 200 aircraft-triggered lightning strikes, 54 recoil events were found to be associated with microsecond bursts of X-rays. The majority of the bursts consist of 1–3 X-ray pulses, with some bursts containing as many as 29 X-ray pulses. We compare the observed superposed X-ray spectrum with modeled spectra using a GEANT4 model of the detector and aircraft, to determine the source potential needed to accelerate the electrons that produce the observed X-rays. A model of the recoil leader was made to determine the gap distance and gap potential between the recoil leader and the aircraft. From the modeling, we determine a solution space for the gap and leader lengths where the gap length is constrained by the observed minimum and maximum times between the onset of the X-ray pulses and the onset of the current pulses detected at the aircraft (1 to 93 m). We also find two constraints from the fitting of the modeled spectra to the superposed spectrum, limiting the leader length to between 1 and ∼240 m.publishedVersio

Observation of Terrestrial Gamma-Ray Flashes at Mid Latitude

We present a sample of Terrestrial Gamma-ray Flashes (TGFs) observed at mid latitudes by the Atmosphere Space Interaction Monitor (ASIM). The events were detected between June 2018 and August 2020 in the latitude bands between 35° and 51° in both hemispheres, which we hereafter refer to as “mid latitudes.” The sample includes the first observations above urn:x-wiley:2169897X:media:jgrd57293:jgrd57293-math-0001 and consists of 14 events clustered in four geographical regions: north-west Atlantic and eastern USA; Mediterranean Sea; the ocean around South Africa; and north-eastern China and Siberia. We examine the characteristics of each event, both standalone and in the context of the global ASIM TGF data set, and we find that our sample is consistent with the global population concerning the number of counts, but shows significantly shorter durations. We analyze the meteorological context and the general evolution of the parent storms and we show that the storms are not extreme in terms of total duration and extension. Whenever possible, we also include the radio sferics and the peak current of the parent stroke. Finally, we present an estimation of the TGF occurrence rate at mid latitudes, based on ASIM's exposure, the local flash rate and tropopause altitude, and we show that it is outside but very close to two standard deviation from the rate of production at tropical latitudes, corrected by the higher atmospheric absorption of higher latitudes. This means that atmospheric absorption plays a major role in the detection of TGFs at mid latitudes, but we cannot rule out other factors.publishedVersio

Production of Terrestrial Gamma-Ray Flashes During the Early Stages of Lightning Flashes

Terrestrial Gamma-ray Flashes (TGFs) are short emissions of high energy photons associated with thunderstorms. It has been known since the discovery of TGFs that they are associated with lightning, and several case studies have shown that the TGFs are produced at the initial phase of the lightning flash. However, it has not been tested whether this is true in general. By using the largest TGF sample up to date, combined with ground-based radio lightning detection data, we perform a statistical study to test this. One of the TGF missions is the Atmosphere-Space Interactions Monitor (ASIM) consisting of the innovative combination of X- and gamma-ray detectors, optical photometers and cameras. This allows us to investigate the temporal relation between gamma-rays produced by TGFs and the optical signal produced by lightning discharges. Based on stacking analysis of the TGF sample and ground-based measurements of associated lightning activity, together with the high temporal resolution of the optical signal from the ASIM photometers, it is shown that TGFs are produced in the beginning of the lightning flashes. In addition, for a significant fraction of the TGFs, the lightning activity detected in radio is enhanced in an interval between 150 and 750 ms following the TGFs, and is co-located with the lightning associated with the TGFs. The enhanced lightning activity is not evident in a randomly selected sample of flashes. This indicates that the activity between 150 and 750 ms is a characteristic property of a significant fraction of flashes that start with a TGF.publishedVersio

Terrestrial Gamma-Ray Flashes With Accompanying Elves Detected by ASIM

The Atmosphere-Space Interactions Monitor was designed to monitor Terrestrial Gamma-ray Flashes (TGFs) and Transient Luminous Events (TLEs) from space, enabling the study of how these phenomena are related. In this paper, we present observations of 17 TGFs with accompanying Elves. TGFs are short and highly energetic bursts of gamma photons associated with lightning discharges, whereas Elves are TLEs that are observed as concentric rings of ultraviolet (UV) and visible light at ionospheric altitudes, produced by the excitation of N2 molecules when an electromagnetic pulse hits the base of the ionosphere. Elves were identified when optical detections in the UV band could be clearly distinguished from other optical signals from lightning strokes. The TGFs they accompanied had short durations and were associated with particularly high peak current lightning. Lightning sferics associated with these events were detected by the global lightning network GLD360 and the World Wide Lightning Location Network, and they were, with the exception of one event, observed over ocean or coastal regions. It is likely that these events were associated with Energetic In-cloud Pulses. We show that short duration TGFs tend to be associated with higher peak currents than long duration TGFs.publishedVersio

Spectral Analysis of Individual Terrestrial Gamma-ray Flashes Detected by ASIM

The Atmosphere-Space Interactions Monitor (ASIM) is the first instrument in space specifically designed to observe terrestrial gamma-ray flashes (TGFs). TGFs are high energy photons associated with lightning flashes and we perform the spectral analysis of 17 TGFs detected by ASIM. The TGF sample is carefully selected by rigorous selection criteria to keep a clean sample suitable for spectral analysis, that is, suitable count statistics, low instrumental effects, and reliable source location. Monte Carlo modeling of individual TGFs has been compared to the observed energy spectra to study the possible source altitudes and beaming geometries. A careful model of the instrumental effects has been developed and validated. Several combinations of source altitudes and beaming geometries are accepted by the statistical tests for all the TGFs in the sample resulting in a large uncertainty in the estimate of the intrinsic source luminosity. The analyzed TGFs show significant variations in observed fluence independent of the distance between source and ASIM. A lower limit on the maximum photon energy produced by TGFs is estimated to be 24 MeV for the analyzed TGFs. The intrinsic limitations of TGF spectral analysis from space are also investigated and it is found that the ability to constrain the source altitude and beaming geometries of TGFs strongly depends on the distance between source and satellite. With the current generation of instruments with effective areas in the range of few hundreds cm2, it is very difficult to constrain reliably the source properties without the help of simultaneous measurements in the radio band.publishedVersio

Aircraft interaction with electric field of thundercloud and observations of hard radiation

The purpose of this thesis is to study the relationship between the different phases of lightning flashes and the production of high-energy emissions, as well as highenergy emissions observed from thunderclouds. The data consists of local electric field-, current- and energy measurements on board an aircraft. The lightning attachment points on the aircraft, were identified by the local electric field signatures of the positive and negative leader initiations. Showing a preference of attachment points based on aircraft model and/or flight altitude. Four distinct radiation producing events were identified by the electric fields and x-ray counts measured during the flights. Single photons in association with the stepping of negative leaders, with photon energies close to 100 keV. A correlation between the changes in local electric field, current and the energy of the observed photons were investigated, but were not found. Bursts of photons in association with recoils were observed. Recoils are microsecond fast changes in the local electric field, observed during the return strokes, dart leaders and recoil streamers of a lightning flash. The energies of the observed photon bursts are similar to that of the single photons observed during negative leader steps. Minute long gamma-ray glows from the data set has previously been reported. The findings were used as a basis to search for more gamma-ray glows in the dataset, and a total of 7 new low intensity gamma-ray glow candidates were found. A statistical analysis of the count rates were done for all observed gamma-ray glows to compare the events. Millisecond long x-ray count rate enhancements were observed on 14.01.2016. The lightning flashes were analysed by examining x-ray count rates and local electric fields. The energy spectrum of some of the observed flashes revealed an enhancement at 511 keV, which is the signature energy for positron annihilation. Two previously observed events were compared to two new findings, showing a very good correlation